The present invention relates to acoustic antennae equipped with reflectors.
The technical field of the invention is that relating to the construction of such antennae, particularly but not exclusively receiving antennae of sonar devices (underwater ultrasonic direction finders).
Hitherto the receiving antenna of a sonar device has conventionally comprised rows of hydrophones disposed on an acoustically transparent support. In order to improve the directivity of sonar antennae, antennae have been constructed which are equipped with a reflector disposed rearwardly of the hydrophones and permitting suppression of the image lobes. However, such an antenna has a degree of sensitivity which varies as a function of frequency, in such manner that the pass band of the antenna is relatively narrow, i.e. of the order of one octave and a half.
The reflectors employed in submarine acoustics are made from materials having an acoustic impedance which is very different from that of water. These reflectors belong either to the category of hard reflectors, (for example metal reflectors) the impedance of which is several times higher than that of water, or to the category of soft reflectors the impedance of which is very much lower than that of water. Most frequently in submarine acoustics there are employed soft reflectors which have high acoustic impedance rupture or difference with respect to water and thus enhanced reflecting power.
In the following discussion, a description will be given more especially of an antenna comprising a soft reflector and pressure-sensitive hydrophones, this being the most frequently employed arrangement. However, this selection implies no kind of limitation of the invention. The invention may equally well be applied to an antenna comprising a hard reflector and velocity-sensitive hydrophones, for example hydrophones having flexing blades.
Plane acoustic waves of wavelength .lambda., reflected on a soft reflector, give rise forwardly of the reflector to stationary pressure waves which comprise a node on the surface of the reflector and a first antinode at distance .lambda./4 from the reflector. The hydrophones generally employed with such a reflector are pressure sensitive, and if they are all disposed at the same distance d forwardly of the reflector there is obtained maximum sensitivity for wavelength .lambda. o= 4d and a pass band, centred on .lambda. o, the width of which is substantially equal to one octave and a half. The small width of the pass band reduces the importance of this type of antenna equipped with a reflector.
An antenna comprising a row of hydrophones all disposed at the same distance d1 forwardly of a reflector has a maximum sensitivity for a pre-determined frequency f1 = V/.lambda.1 = V/4d 1, V being the velocity of sound in water, i.e. approximately 1,500 meters per second. Thus it would appear to be logical, in order to increase the width of the pass band of the antenna, to construct the antenna with a plurality of rows of hydrophones disposed at distances d1, d2 . . . di forwardly of the reflector, and to simply provide the sum of the signals captured by all the hydrophones located in each plane perpendicular to the reflector surface. However, by proceeding in this manner, no substantial broadening of the pass band is obtained.